Copyright Sams Teach Yourself Shell Programming in 24 Hours

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After login receives your username and password, it looks through the file /etc/passwd for an entry matching the information you provided. If it finds a match, login executes a shell and exits. As an example, on my system the matching entry for my username, ranga, in file /etc/passwd is: ranga:x:500:100:Sriranga Veeraraghavan:/home/ranga:/bin/bash As you progress through the book, I will explain the information stored here. Note - For those readers who are not familiar with UNIX files or filenames such as /etc/passwd, this topic is covered extensively in Chapters 3, "Working with Files," and 4, "Working with Directories." I will discuss files briefly in this chapter. A general idea from other operating systems of what files are is enough to understand these examples. If no match is found, the login program issues an error message and exits. At this point the getty program takes over and displays a new login prompt. The shell that login executes is specified in the file /etc/passwd. Usually this is one of the shells that I covered in the previous chapter. In this book I assume that the shell started by the login program is /bin/sh. Depending on the version of UNIX you are running, this might or might not be the Bourne shell: ● On Solaris and FreeBSD, it is the Bourne shell. ● On HP-UX, it is the POSIX shell. ● On Linux, it is the Bourne Again shell. Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 2: Script Basics Sections in this Chapter: Previous Chapter Next Chapter The UNIX System Summary Shell Initialization Questions Getting Help Terms Previous Section Next Section © Copyright Macmillan Computer Publishing. All rights reserved.
Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 2: Script Basics Sections in this Chapter: Previous Chapter Next Chapter The UNIX System Summary Shell Initialization Questions Getting Help Terms Previous Section Next Section Shell Initialization Interactive Versus Noninteractive Shells Making a Shell Script Executable Initialization File Contents When the login program executes a shell, that shell is uninitialized. When a shell is uninitialized, important parameters required by the shell to function correctly are not defined. The shell undergoes a phase called initialization to set up these parameters. This is usually a two step process that involves the shell reading the following files: ● /etc/profile ● profile The process is as follows: 1. The shell checks to see whether the file /etc/profile exists. 2. If it exists, the shell reads it. Otherwise, this file is skipped. No error message is displayed. 3. The shell checks to see whether the file .profile exists in your home directory. Your home directory is the directory that you start out in after you log in. 4. If it exists, the shell reads it; otherwise, the shell skips it. No error message is displayed. As soon as both of these files have been read, the shell displays a prompt: $ This is the prompt where you can enter commands in order to have them execute. Note - The shell initialization process detailed here applies to all Bourne type shells, but some additional files are used by bash and ksh. You can obtain more information about this process for a particular shell using the man command explained later in this chapter. Interactive Versus Noninteractive Shells When the shell displays a prompt for you, it is running in interactive mode.
Page 1 and 2: Sams Teach Yourself Shell Programmi
Page 3 and 4: ● Using Single Quotes ● Using D
Page 5 and 6: ● Questions Hour 20: Debugging
Page 9 and 10: ● Part I is an introduction to UN
Page 11 and 12: Many of the examples and the answer
Page 17 and 18: $ date [ENTER] Wed Dec 9 08:49:13 P
Page 19 and 20: anga pts/0 Dec 9 08:49 $ Here, the
Page 23 and 24: The original UNIX shell was written
Page 25 and 26: There are three major versions of k
Page 31: example, when you execute the comma
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Page 37 and 38: chmod a+x ./logins Here you are usi
Page 41 and 42: Try using the man command to get mo
Page 49 and 50: Hidden Files So far you have used l
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Page 53 and 54: Numbering Lines The cat command als
Page 55 and 56: command $ wc -w .rhosts produces th
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Sams Teach Yourself Shell Programmi
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$ ls -ld /home/ranga This produces
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You can see the relative path by us
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Sections in this Chapter: File Type
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The permission block for this file
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Here, expression is a statement of
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For example, if the directory pub c
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Restrictions The super user, root,
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Running the same ls command as in t
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Tip - Background Processes That Wri
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the fg command. By default, the pro
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dhorvath 6770 3662 2 10:35:45 pts/6
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By default, a process runs with the
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Process Identifier --The process id
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Variable Names The name of a variab
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Array Variables The Bourne shell, s
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$ band[2]=mike $ band[3]=gene Tip -
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Hour 7: Variables Sections in this
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uses internally. Users can modify t
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Here, command is the name of a comm
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$ ls ch??.doc to accomplish this. M
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© Copyright Macmillan Computer Pub
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is unset. Assigning a Default Value
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© Copyright Macmillan Computer Pub
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It now gives this error result: Hel
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Sections in this Chapter: Quoting w
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To the shell, one or more spaces or
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Quoting Regular Expression Wildcard
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fi list5 Both the elif and the else
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Here you have an elif statement tha
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Consider the following if statement
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If you were to use the - n option i
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else echo "An error was encountered
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1. First the test [ -z "$DTHOME" ]
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esac Here the string contained in $
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The execution of a while loop proce
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5 4 3 2 1 0 6 5 4 3 2 1 0 7 6 5 4 3
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2. If the exit status of command is
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for i in 0 1 2 3 4 5 6 7 8 9 do ech
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3. When the user enters a value, $R
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The continue Command The continue c
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If you want to list the contents of
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In this version, the basename comma
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USAGE="Usage: ´basename $0´ [-c|-
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uuencoding a file. For readers who
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fi fi echo $MSG You could simplify
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● Formatting escape sequences Emb
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Name User Name Sriranga ranga Sriva
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Table 13.2 Formatting Sequence Type
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printf "%-32s " "$i" if [ -d "$i" ]
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anga ttyp0 Oct 27 19:42 (:0.0) amma
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Say you need to send a list of phon
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"Miscellaneous Tools," I will show
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$ exec 4>fd4.out associates the fil
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command > file 2>&1 list > file 2>&
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when the while loop exits, the valu
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Note - The shell uses the value of
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This function has three important p
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} OLDIFS="$IFS" IFS=: # print each
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} # set the directory to pop to the
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index.html RCS humor misc downloads
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unix has several additional pieces
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pqops.c:/* Function to delete a nod
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Here I specified set2 as the space
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$ tr '!?":;\[\]{}(),.\t\n' ' ' < /h
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in 69 ' 65 the 389 to 164 of 127 No
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tr '[:classname:]' 'set2' Here clas
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single letters, whereas in awk the
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able was I, ere I saw elba Because
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By adding the ^ metacharacter as fo
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\$[0-9]*\.[0-9][0-9]\/[a-zA-Z]*/ If
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Here you list the name of a fruit a
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$ rm fruit_prices.txt.$$ Performing
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Fortunately, the s command provides
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lrwxrwxrwx 1 ranga users 8 Nov 27 1
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The original awk has remained almos
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Kiwi 22 Pineapple 35 Apple 78 All t
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Pineapple $1.29 35 REORDER Apple $0
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$ ls -l produces output formatted s
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The value that you assign a variabl
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x=x+1 In awk this can be written in
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the pattern specifies. To solve you
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awk -F: '{ ... }' The major differe
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identical to expressions you looked
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The basic syntax is do { actions }
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B:amount Here amount is the balance
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The OUTPUT variable contains the >
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: ${LINES:=24} ${TERM:?"TERM not se
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Sections in this Chapter: The eval
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eports/1998 The starting directory.
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find: -type Option The -type option
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find / -name alpha -size +50 -mtime
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18 quit $ obase=16 sets the output
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The default action for the signals
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fi Init() { printf "INFO: Initializ
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4. Wait for the program to finish e
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} if [ $? -eq 0 ] ; then echo "Sub-
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A third way of enabling debugging i
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Here the debugging mode is enabled
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if [ $YN = "y" ] ; then Making this
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$ /bin/sh -nv ./buggy2.sh the outpu
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Deleting old backups, please wait..
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If you do not type a response and p
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To deactivate debugging, use either
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3 MSG="hello" 4 echo_error $MSG Her
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echo "ERROR: File $MYFILE was not f
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previous call to this function. Aft
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} Before you look at some examples
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dev/hda1 1190014 664661 463867 59%
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The number returned is in kilobytes
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getUID() { id $1 | sed -e 's/(.*$//
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63 printf "$1 (y/n)? " 64 65 if [ -
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}' 171 # Args: $1 -> the command na
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Some UNIX versions implement a work
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24 printERROR "The source $1 is not
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easily modify the script to replace
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Hour 22: Problem Solving with Shell
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To maintain your address book, you
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produces two matches: Name: Srirang
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26 # enable erasing input 27 28 stt
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2. Based on user feedback, delete t
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73 promptYESNO "Delete this entry"
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MYADDRESSBOOK="$HOME/addressbook" i
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Here, options is one or more of the
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Now that you have looked at using t
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Abstraction is a technique used to
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warning: '-' deprecated; use 'ps au
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produce the same result if the curr
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The structure of the UNIX directory
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OLDPREFIX=aaa NEWPREFIX=bbb for FIL
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history (Korn/Bash) shows the most
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Sections in this Chapter: Reserved
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^ bitwise exclusive OR operation |
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Bash array initialization USERVAR=(
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order to fix problems. default beha
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minor number UNIX uses this to asso
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important parameters. This is usual
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Sections in this Chapter: Chapter 1
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USAGE="Usage: ´basename $0´ [-v]
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Appendix C: Quiz Answers Sections i
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awk -F: ' $1 == "B" { BAL=$NF ; nex
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} if [ $DEF_TOUT -ne 0 ] ; then sle
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Sections in this Chapter: Chapter 1
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} fi if [ ! -d "$1" ] ; then printE
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numeric variables, 1st pattern-spec
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wildcards, 1st Bourne shells. See s
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(colon), 1st character, 1st, 2nd, 3
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grep, 1st, 2nd line numbers, 1st li
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stty addperson script, 1st tail, 1s
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emoving temporary files, 1st displa
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mv command, 1st, 2nd rmdir command,
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interactive mode, 1st multiple inpu
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starting directory, 1st, 2nd findin
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installation scripts disk usage, 1s
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eak command, 1st, 2nd, 3rd continue
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mvdir.sh, 1st, 2nd, 3rd, 4th, 5th,
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edirection, 1st, 2nd appending to f
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[0-9]*, 1st percent sign (%) job nu
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profiles shell specific startup wit
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hardware type, 1st, 2nd improving,
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(standard error), 1st displaying me
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prompt, 1st, 2nd, 3rd, 4th quoting
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egp command, 1st -k option sort com
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Index A B C D E F G H I J K L M N O
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